Using VideoTexture.ImageRender to get camera image
Okay. I looked into the source code of Blender and it seems that ImageRender only works when rendering to a texture and it is not the currently active camera.

What I did was to add an additional plane, create a material along with a texture rendered it to that plane. After that, I was able to store the image.

In addition to that, I set the new camera as the active camera for the scene. Hope that this answer will help other people.